1. Field of the Invention
The present invention relates to communication systems, and more particularly, to a communication system that performs a communication control.
2. Description of the Related Art
Recently, an optical subscriber system having an optical subscriber communication network has been proposed. Particularly, there has been considerable activity in the construction of a PON (Passive Optical Network) system mainly employed in the Europe, and an ATM-PON, which utilizes an ATM (Asynchronous Transfer Mode) in order to guarantee a communication band and a requested quality for real-time communication of audio and video. Such an optical access system is an essential technique for providing a large capacity of communication services at a low cost, such as video-on-demand, CATV and high-speed computer communication, and attracts a lot of attention as a key network of the next generation.
FIG. 22 is a diagram of an ONU (Optical Network Unit) and an SLT (Subscriber Line Terminal) that construct an optical access system. Referring to FIG. 22, a plurality of ONUs 400-1 through 400-n are installed at subscribers' homes, and an SLT 500 is installed on an office side.
The ONUs 400-1 through 400-n and the single SLT 500 are connected by an optical transmission path via a branch part 300, and shares the optical transmission path by time-division control using polling. That is, each ONU is allowed to send information to the SLT after receiving a transmission acknowledgement from the SLT.
Each of the ONUs 400-1 through 400-n is equipped with a buffer B, which stores information to be sent to the office (up information). Further, each of the ONUs 400-1 through 400-n is required to notify the SLT 500 of the storage status showing how much up information is currently stored in the buffer B.
The SLT 500 receives the above notice, and recognizes the storage status of the buffer B of each of the ONUs 400-1 through 400-n. Then, the SLT 500 allocates the bands to the ONUs 400-1 through 400-n so that the ONUs can send up information to the SLT 500. The above buffer management control achieves transfer of information/data between the ONUs 400-1 through 400-n and the SLT 500.
A particular slot is exclusively used to send storage status information to the SLT 500 when the ONUs 400-1 through 400-n notify the SLT 500 of the storage status of their buffers B. However, the exclusive use of the particular slot degrades the efficiency of transfer of user information that is real information.
Japanese Unexamined Patent Publication No. 10-242981 proposes to use an idle field in the overhead added to each cell and make a notice of the storage status of one buffer at one time. However, presently, the ITU-T Recommendation G. 983 gives definitions to all the fields of the overhead. More particularly, the up transmission uses the overhead that consists of a guard field for avoiding collision of two cells, a bit synchronization field for establishing a bit-based synchronization, and a byte synchronization field for establishing a byte-based synchronization. If some existing fields are omitted, the originally needed functions will be lost and the communication quality will be degraded.